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Herbaceous Layer Species Richness of Southeastern Forests and Woodlands: Patterns and Causes

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This chapter examines species richness patterns in the herbaceous layer vegetation of forests and woodlands of southeastern North America. It demonstrates the changing importance of various drivers of species richness across environmental and geographic gradients and across vegetation types, and shows how the relative importance of those drivers varies with scale of observation. The most important processes structuring species diversity patterns in southeastern forests and woodlands appear to be cation availability (environmental favorableness), disturbance (flooding, fire, grazing), mass effects, and the relative size of the species pool.
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... Herbaceous taxa represent more than 80 percent of plant species in temperate forests with high densities and diversities of these herbs co-occurring within small areas (Gilliam 2007;Peet et al. 2014). Spatial resource partitioning and environmental heterogeneity are the most widely cited mechanisms facilitating coexistence among forest herbs (Bell et al. 2000;Bartels and Chen 2010;Beatty 2014;Catella et al. 2019;Beck and Givnish 2021). ...
... Spatial resource partitioning and environmental heterogeneity are the most widely cited mechanisms facilitating coexistence among forest herbs (Bell et al. 2000;Bartels and Chen 2010;Beatty 2014;Catella et al. 2019;Beck and Givnish 2021). Forest herb distributions vary in response to environmental variation at a variety of spatial scales (Curtis 1959;Struik and Curtis 1962;Beatty 2014;Peet et al. 2014). At local scales, differential plant responses to fine-scale variation in soil depth (Bratton 1976;Beck and Givnish 2021), soil fertility and soil moisture (Collins et al. 1984;Crozier and Boerner 1984;Vellend et al. 2000), microtopography (Beatty 1984;Peterson et al. 1990), light availability (Anderson et al. 1969;Thompson 1980), and other abiotic factors can promote coexistence via spatial resource partitioning. ...
... At local scales, differential plant responses to fine-scale variation in soil depth (Bratton 1976;Beck and Givnish 2021), soil fertility and soil moisture (Collins et al. 1984;Crozier and Boerner 1984;Vellend et al. 2000), microtopography (Beatty 1984;Peterson et al. 1990), light availability (Anderson et al. 1969;Thompson 1980), and other abiotic factors can promote coexistence via spatial resource partitioning. Yet, spatial resource partitioning cannot account for coexistence among many functionally similar species at small spatial scales (Gilliam 2007;Beatty 2014;Peet et al. 2014;Beck 2020). Potential interactions among herbaceous plants and soil biota could shape many understory community dynamics and potentially promote local coexistence, but these require further study in temperate forest understories (Whigham 2004;Comita et al. 2014). ...
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Antagonistic interactions between plants and soil biota promote species diversity in many plant communities but little is known about how these plant–soil interactions influence herbaceous species in temperate forests. To assess the potential for soil biota to affect the growth of forest herbs, I conducted a greenhouse experiment in which seedlings of nine focal herb species common in Wisconsin (USA) forests were grown in soil derived from conspecific and heterospecific plants. This soil origin treatment was crossed with a subsequent treatment in which half of the soils were pasteurized to eliminate soil biota. The presence and origin of soil biota had variable effects on plant growth among the nine focal species. Thalictrum dioicum, Elymus hystrix, and Solidago flexicaulis growth were inhibited by the presence of soil biota in unpasteurized soils. Thalictrum dioicum seedlings grown in conspecific, unpasteurized soil accumulated 30% less biomass than seedlings grown in heterospecific, unpasteurized soil indicating that host-specific effects of microbial pathogens restrict seedling growth. Similarly, E. hystrix seedlings were 11% smaller in conspecific-trained soils. The remaining herb species showed no significant response to experimental treatments manipulating soil biota. These variable growth responses highlight the potential for differences in plant–soil interactions among plant species to influence local plant distributions and community dynamics. Janzen–Connell effects, like those observed in T. dioicum and E. hystrix, could promote coexistence among certain species and contribute to high local plant diversity in temperate forest understories.
... This prediction of suppression of tree seedlings by dense herbs under closed forests on sites with rich soils has been suggested by several authors (e.g., Graves et al., 2006;Maguire & Forman, 1983;Peet & Christensen, 1988;Peet, Palmquist, & Tessel, 2014;Wardle, 1959). ...
... Competitive exclusion of tree recruits by herbs is not a general rule, as it may be prevented by factors that limit herb density or height, such as grazing, chronic fire, low soil fertility or allelopathy (Peet et al., 2014). Moreover, where herbs are sparse, tree seedlings are not necessarily abundant. ...
... For instance, both herb layer cover and height are expected to be positively related to soil nutrients and overstorey openness. With regard to a generalized Gaussian response curve, the distribution of herb cover along a soil fertility gradient tends to be strongly negatively skewed, at least within forests (Peet et al., 2014). ...
Article
Competitive inhibition of temperate forest tree recruits by herbs is likely important on sites with high fertility owing to faster height growth and consequent preemption of light. We explored the site conditions and stand structure under which herbaceous growth has an impact on tree regeneration. Plot data from 610 forest sites were collected from five areas across the southern Appalachian Mountains. Several plant guilds were distinguished based on various biological traits. Deterministic models of forest understory were validated through recursive path analysis. The numerical analyses were performed both on all plots and on a subset of 150 plots free of evergreen shrubs. In general, total herb cover increased with soil fertility, but in sites without evergreen shrubs no relationship emerged. Total herb cover varied inversely with woody stem density (saplings excluded), but the slope was much less steep in the absence of evergreen shrubs. Tree sapling density displayed a left‐tailed, asymmetric response with respect to total herb cover, but a symmetric unimodal response against tall herb cover. The shape of the distribution of tree stems by diameter class shifted from unimodal under very sparse herbaceous layer to negative exponential in stands with mid to high herb cover. This was due to the suppressive impact of evergreen shrubs on understory vegetation, which led to a positive covariance between total herb cover and tree sapling density. These two understory variables became unrelated in the path model built on the subset without evergreen shrubs, but a similar model involving tall herbs revealed a direct negative effect of tall herb cover on tree sapling density. Our results provide evidence of tree recruits exclusion by tall herbs on fertile sites but not on acidic sites, where herb interference is much reduced by the suppressive effect of evergreen shrubs and trees on herbaceous layer vegetation.
... Therefore, it is not surprising that richness would increase over the year following the fire event, especially as pulses of seeds from the species pool could be increasing dispersal following fire disturbances (Harms et al., 2017;L. K. Kirkman et al., 2016;Peet et al., 2014). Alternatively, in sites with longer time since fire, such as 4 or 10 years, we would expect richness to decrease as woody shrubs begin to dominate, and this trend has been documented in other studies (Beckage et al., 2009;Palmquist et al., 2014;Peet et al., 2018). ...
... Although factors such as elevation gradients and soil composition, texture and moisture are important edaphic factors influencing plant cover, richness and overall community composition (Carr et al., 2009;Drewa et al., 2002b;L. K. Kirkman et al., 2001;Mitchell et al., 1999;Peet, 2006;Peet et al., 2014), unfortunately, soil texture and moisture were not measured in this study. Within our xeric study sites, the unexplained variation in above-ground biomass, wiregrass cover and species richness (Figure 3) is probably due to differences in soil moisture. ...
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Restoration of ecosystems is complex, with multiple targets that can work in concert or conflict with each other, such as biodiversity, species dominance, and biomass. When properly managed, longleaf pine (LLP) savannas are among the most biologically diverse habitats in the world. However, anthropogenic influences, such as fire suppression, have decimated this ecosystem and its biodiversity, making restoration a priority. Here, we describe the biodiversity and community dynamics seen in the understory layer across xeric LLP savannas in North Carolina (NC) and then answer the following questions: What are the predictors of (1) biodiversity, (2) dominance, and (3) biomass at multiple spatial scales? Fifteen observational study sites in North Carolina spanning from the Sandhills to the Coastal Plain. At each of the fifteen sites, twenty‐five sampling plots were established where aboveground herbaceous biomass, species presence and abundance, soil characteristics, and light availability were measured along with numerous other environmental variables. Considerable variation exists across study plots within and across sites, with plant species richness ranging from 1–17 per m2. The relative cover of the dominant grass species, Aristida stricta (wiregrass), also varied greatly within and across sites, with a median of ~30% relative cover per plot. Wiregrass was a significant predictor of biomass and biodiversity at small scales. With increasing wiregrass abundance, richness decreases, with 25% relative wiregrass cover leading to the highest levels of biodiversity. Likewise, as wiregrass abundance is one of the stronger predictors of aboveground biomass, we also found a unimodal richness‐biomass relationship. Our results indicate that at lower ends of the productivity and richness gradients, land managers can increase all three restoration targets in the understory at the same time; however, at more diverse and productive sites, restoration practitioners may need to prioritize one target or find a balance between all three.
... There have been a multitude of studies assessing the connection between resource availability and herbaceous layer species richness in temperate forests (including forbs; e.g., Bellemare et al., 2005;Burton et al., 2011;Lundholm, 2009;McEwan & Muller, 2011;Peet, Palmquist, & Tessel, 2014). Recent studies have also highlighted the importance of biotic processes in forest forb communities, like dispersal limitation (Burton et al., 2011;Ehrlén & Eriksson, 2000;Flinn & Vellend, 2005;Pärtel, Szava-Kovats, & Zobel, 2011) and the role of plant-soil feedbacks (Burke, Klenkar, & Medeiros, 2018). ...
... Forb richness also increased with increasing calcium availability in both seasons and at both spatial scales ( Figures 4a,b and 5a,b). The strength and persistence of this particular trend throughout our study, as well as its prevalence in the literature (Bellemare et al., 2005;Burton et al., 2011;McEwan & Muller, 2011;Peet et al., 2014), suggests it is worth further consideration. We note that mean calcium is also highly correlated with pH and potassium variability ( Figure 3). ...
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While plant community theory tends to emphasize the importance of abiotic heterogeneity along niche axes, much empirical work seeks to characterize the influence of the absolute magnitude of key abiotic variables on diversity. Both magnitude (as reflected, e.g., by a mean) and heterogeneity (variance) in abiotic conditions likely contribute to biodiversity patterns in plant communities, but given the large number of putative abiotic drivers and the fact that each may vary at different spatiotemporal scales, the challenge of linking observed biotic patterns with the underlying environment remains acute. Using monitoring data from a natural resource agency, we compared how well statistical models of the mean, heterogeneity, and both the mean and heterogeneity combined of 17 abiotic factor variables explained patterns of forb species richness in Northeast Ohio, USA. We performed our analyses at two spatial scales, repeated in spring and summer across four forest types. Although all models explained a great deal of the variance in species richness, models including both the mean and heterogeneity of different abiotic factors together outperformed models including either the mean or the heterogeneity of abiotic factors alone. Variability in forb species richness was mostly due to changes in mean calcium levels regardless of forest type. After accounting for forest type, we were able to attribute variation in forb species richness to changes in the heterogeneity of different abiotic factors as well. Our results suggest that multiple mechanisms act simultaneously according to different aspects of the abiotic environment to structure forb communities, and this underscores the importance of considering both the magnitude of and heterogeneity in multiple abiotic factors when looking for links between the abiotic environment and plant community patterns. Finally, we identify novel patterns across spatial scales, forest types, and seasons that can guide future research in this vein. Open research badges: This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.kp3cb17.
... Herb layers are usually sparse, with only a few shade-tolerant species frequent, and then usually with low cover. In addition to reduced insolation, the low level of basic cations and typically dry upland soils limit the herb layer cover and diversity (Peet et al. 2014;Hakkenberg et al. 2020); in some places a high population density of white-tailed deer further limits development of both shrub and herb cover due to heavy browsing. ...
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The Piedmont (PDMT) ecoregion of the USA stretches from New Jersey to Alabama, nestled between the Coastal Plain and Blue Ridge Mountain physiographic provinces. Many of the notable Piedmont plant communities, including the dominant oak-hickory forests of the region, are reliant upon fire to some degree. Before human settlement, most Piedmont vegetation burned relatively frequently and at low intensities, resulting in extensive closed canopy oak-hickory forests, studded with patches of open woodland and savanna largely defined by unusual soil conditions. Indigenous peoples of the Piedmont used fire as a land management tool for both agriculture and game production. Historical changes in land use throughout the region have altered fire regimes and changed forest dynamics dramatically over the past 400 years. Euro-American settlement led to widespread clearing of land for agriculture and logging; by the early twentieth century, very little old-growth forest remained in the Piedmont. During the mid-twentieth century, the decline of agriculture and the aggressive suppression and exclusion of wildfires brought about the growth of successional forests in the place of older, fire-mediated communities. The Piedmont region is currently experiencing a rapid expansion of the human population and land development, making restoration of the historical fire regime a challenge. However, land managers frequently do use prescribed fire to enhance timberland and restore rare plant communities.
... Riparian shrubland with Alnus serrulate and Xanthorhiza simplicissima growing along Blue Ridge Mts. rivers are probably the most species-rich woody communities in temperate North America, harboring as many as 129 plant species per 100 m 2 (Peet et al., 2014). Thus, understanding the drivers and spatial patterns of this extraordinary species richness is of vital importance. ...
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This study aimed to analyze plant species richness in riparian forests at both local and regional scales across several watersheds in the Sudetes (Poland, Central Europe). Specifically, species richness in riparian forest was compared to other forest types in the same region. It was also hypothesized that due to high complexity and dynamics, riparian forests share a higher number of rare species. In addition, the longitudinal pattern of species richness was analyzed at both local and regional scales. Finally, the effect of topography on species richness in riparian forests in spring areas and along rivers of various sizes was analyzed. Riparian forests have significantly higher alpha diversity than beech and ravine forests, but oak forests showed a similar level of diversity. However, a comparison of accumulation curves showed, that riparian forests are the most species-rich at a regional scale. All forest types had a similar share of rare species. Eight uniform groups of rare species were distinguished in riparian forests and reflected the riparian complexity and dynamics. The number of plant species per plot was highest in spring areas and decreased from headwaters to lower reaches. The estimated total number of species showed a similar pattern; however, the highest number was estimated for riverine forests along 3rd order streams and therefore suggest a unimodal pattern of gamma diversity along a longitudinal (upstream–downstream) gradient. The effect of topographic variables on species richness differed depending on the position in the river network.
... Pine savanna ecosystems of the southeastern USA are extremely biodiverse (Engstrom 1993, Lubertazzi and Tschinkel 2003, Means 2007, Noss 2013, Peet et al. 2014), yet extremely threatened (Frost 2007). In much of the remaining southeastern pine savanna, the canopy and understory vegetation have been greatly altered since European settlement, largely from fire suppression, plowing, and hardwood encroachment (Van Lear et al. 2005). ...
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Where historical fire regimes have been disrupted, reduction in woody vegetation is often used to maintain or restore habitat for grassland and early successional birds. In pine savanna ecosystems of the southeastern USA, mechanical hardwood canopy reduction can restore pine savanna communities and is often employed on privately owned lands to improve habitat for the Northern Bobwhite (Colinus virginianus), although scant empirical evidence exists of its effects on target or non‐target species. We measured the response of a pine savanna specialist, the Bachman's Sparrow (Peucaea aestivalis), to large‐scale hardwood reduction in a before–after–control–impact design on two properties where two‐year fire‐return intervals were established and the Bachman's Sparrow population was stable. We investigated the effects of mechanical hardwood reduction on Bachman's Sparrow daily nest survival, cause‐specific nest mortality and adult male annual survival. During the four‐year study, we monitored 107 Bachman's Sparrow nests, recorded 49 nest predation events, and banded 113 adult male Bachman's Sparrows. We found Bachman's Sparrow nest and adult survival were resilient to changes in the hardwood canopy and did not differ significantly between treatment and control sites. Average annual adult male survival was 0.41 (0.32–0.52) and daily survival rate of nests with surveillance declined annually from 0.94 (0.92–0.96) to 0.88 (0.83–0.92). The identity of predators at nests was dominated by two snake species, black racer (Coluber constrictor) and corn snake (Pantherophis guttata). We found evidence for opposing treatment effects on the frequency of nest depredations by the dominant species; racers responded positively and corn snakes responded negatively. Our results suggest a moderate midstory canopy does not limit Bachman's Sparrow vital rates when management includes frequent prescribed fire. Our results also suggest hardwood reduction to mitigate nest predation may be complicated with a diverse predator suite.
... Our finding that plant species richness is positively correlated with soil fertility (H1) is corroborated by several studies in southeastern US forests where soil fertility has been variously characterized by soil cation availability (e.g. Peet et al. 2014), cations like Ca and Mg (Peet and Christensen 1988), soil pH (Costanza et al. 2011), or soil minerals like Mn (Reed et al. 1993). This relationship between soil fertility and species richness is partially explained by the positive effect of total versus relative energy inputs on diversity. ...
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The goal of elucidating the primary mechanisms constraining the assembly and distribution of biodiversity remains among the central unresolved challenges facing the field of ecology. Simulation studies and experimental manipulations have focused on how patterns in community assembly result from bivariate relationships along productivity or environmental gradients. However, the joint influence of multiple resource gradients on the distribution of species richness in natural communities remains under‐studied. Using data from a large network of multi‐scale vegetation plots across forests and woodlands of the southeastern US, we find significant evidence for the scale dependent, joint constraints of forest structure and soil resources on the distribution of vascular plant species richness. In addition to their significant partial effects on species richness, understory light levels and soil fertility positively interact, suggesting a trade‐off between the two limiting resources with species richness peaking both in high‐light, low‐fertility conditions as well as low‐light, high‐fertility settings. This finding provides a novel perspective on the biodiversity‐productivity relationship that suggests a transition in limiting resources from soil nutrients to light availability when enhanced productivity results in reduced light resources for subordinate individuals. Results likewise have meaningful implications for our understanding of scale dependent community assembly processes as size‐asymmetric competition replaces environmental filtering as the primary assembly mechanism structuring temperate forest communities along an increasing soil fertility gradient.
Article
Premise: Environmental heterogeneity influences plant distributions and diversity at several spatial scales. In temperate forests, fine-scale environmental variation may promote local coexistence among herbaceous species by allowing plants to spatially partition microsites within forest stands. Here we argue that shallow soils, low soil water-holding capacity and fertility, and reduced light near tree boles should favor short, shallow-rooted, evergreen species like Anemone acutiloba with low moisture, nutrient, and light requirements. Farther from trees, richer, deeper soils should favor taller, deeper-rooted herbs with greater moisture and nutrient demands, such as Sanguinaria canadensis and Trillium flexipes. Methods: We tested these hypotheses by mapping the fine-scale distributions of Anemone, Sanguinaria, and Trillium individuals within a 50 × 50 m plot, comparing local species' distributions with respect to soil depth and proximity to neighboring trees, and characterizing intraspecific and interspecific spatial associations. Results: Local plant distributions were consistent with our predictions based on leaf height, physiology, and phenology. Anemone was found in microsites on shallower soils and closer to trees than either Sanguinaria or Trillium. In all three species, individual plants were spatially aggregated within 2 m, but spatially segregated from individuals of the other species beyond 2 m. Conclusions: Differential plant responses to fine-scale environmental heterogeneity and observed spatial associations suggest that local species-environment associations could facilitate coexistence. These findings illustrate how fine-scale environmental heterogeneity coupled with phenological and physiological differences likely contribute to spatial niche partitioning among spring-flowering forest herbs and maintain high local plant diversity within temperate forests.
Chapter
Although edaphic variation is common in natural systems, and has often been described as a major driver of plant species diversity, the effect of this edaphic variation on plant diversity has not been described in a comprehensive, synthetic fashion. Understanding this variation is essential, however, as soil nutrients are important drivers of plant community structure. This study takes advantage of multi-scale vegetation sampling along with plot-level soil data from the Carolina Vegetation Survey to examine the relationships between soil nutrients and diversity in forests and woodlands at multiple spatial scales and across floristic regions. We find that there is greater variation in soil characteristics that predict diversity between regions than across scales within regions. In Atlantic Coastal Plain longleaf-pine communities, nitrogen, sulfur, iron, soil pH, organic matter, and silt are important predictors of diversity. In the Fall-line Sandhill longleaf-pine communities of the Carolinas, manganese, nitrogen, soil pH, and silt are the measured variables that predict diversity best. In longleaf-pine communities of Florida, soil pH, iron, nitrogen, and silt are consistently the strongest indicators across all scales from 0.01 to 1000 m ² . In southern Appalachian Mountain forest communities, soil pH, manganese, and calcium are the best diversity indicators. By tailoring models to individual regions, soil characteristics can predict between 39 and 54 % of the variance in diversity at the 0.1 ha scale.
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